High alumina insulators are useful for situations where a high voltage difference must be sustained between conductors which must be physically connected while maintaining a clean vacuum environment. It is desirable to improve the voltage hold off capability of ceramic insulators because improved voltage hold off capability means a smaller size insulator can be used for any given required hold off. Thus, equipment utilizing such insulators can be smaller, capable of higher performance, more efficient and the like. It is also frequently desirable to minimize the buildup of electric charge on an insulator's surface, because such a charge can distort electric fields near the insulator and adversely affect the performance of the device incorporating the insulator, even though high voltage breakdown does not occur